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The Anisoplanatic Point-Spread Function in Adaptive Optics
The effects of anisoplanatism on the adaptive optics point-spreadfunction are investigated. A model is derived that combines observationsof the guide star with an analytic formulation of anisoplanatism inorder to generate predictions for the adaptive optics point-spreadfunction at arbitrary locations within the field of view. The analyticformulation captures the dependencies of anisoplanatism on aperturediameter, observing wavelength, angular offset, zenith angle, andturbulence profile. The predictions of this model are compared tonarrowband 2.12 and 1.65 μm images of a 21" binary (mv =7.3, 7.6) acquired with the Palomar adaptive optics system on the 5 mHale Telescope. Contemporaneous measurements of the turbulence profilemade with a DIMM/MASS (differential image motion monitor/multiaperturescintillation sensor) unit are used together with images of the primaryto predict the point-spread function of the binary companion. Predictedcompanion Strehl ratios are shown to match measurements to within a fewpercent, whereas predictions based on the isoplanatic angleapproximation are highly discrepant. The predicted companionpoint-spread functions are shown to agree with observations to 10%.These predictions are used to measure the differential photometrybetween binary members to an accuracy of 1 part in 103, andthe differential astrometry to an accuracy of 1 mas. Errors in thedifferential astrometry are shown to be dominated by differentialatmospheric tilt jitter. These results are compared to other techniquesthat have been employed for photometry, astrometry, and high-contrastimaging.
| First Results from the CHARA Array. III. Oblateness, Rotational Velocity, and Gravity Darkening of Alderamin
We present observations of the A7 IV-V star Alderamin (α Cep, HR8162, HD 203280) from the Georgia State University CHARA Array. Theseinfrared interferometric angular size measurements indicate anoncircular projected disk brightness distribution for this known rapidrotator. The interferometric observations are modeled as arising from anelongated rigid atmosphere, with apparent polar and equatorial radii ofrp=0.6753+0.0119-0.0135 andre=0.8767+0.0293-0.0183 mas,respectively, for a difference of 201+/-32 μas, and with an axialratio of re/rp=1.298+/-0.051. Using the Hipparcosdistance of 14.96+/-0.11 pc, these angular measures translate to2.18+/-0.05 and 2.82+/-0.10 Rsolar. The inclination ofAlderamin to the line of sight indicated by this modeling is effectivelyedge-on (i=88.2+1.8-13.3 deg). The star has a truerotational velocity of 283+/-10 km s-1 (~83% of breakupvelocity) and a polar temperature of roughly 8400 K. Significantly, anecessary aspect of this modeling is a determination of thegravity-darkening coefficient, which at a value ofβ=0.084+0.026-0.049 is consistent with aconvective photosphere, as expected for an A7 IV-V star. Our detailedcharacterization of this object allows us to investigate variousscenarios for the angular momentum history of Alderamin and theappropriateness of certain stellar evolution models.
| Resolving the Effects of Rotation in Altair with Long-Baseline Interferometry
We report the successful fitting of a Roche model, with a surfacetemperature gradient following the von Zeipel gravity darkening law, toobservations of Altair made with the Navy Prototype OpticalInterferometer. We confirm the claim by Ohishi et al. that Altairdisplays an asymmetric intensity distribution due to rotation, the firstsuch detection in an isolated star. Instrumental effects due to the highvisible flux of this first magnitude star appear to be the limitingfactor in the accuracy of this fit, which nevertheless indicates thatAltair is rotating at 0.90+/-0.02 of its breakup (angular) velocity. Ourresults are consistent with the apparent oblateness found by van Belleet al. and show that the true oblateness is significantly larger owingto an inclination of the rotational axis of ~64° to the line ofsight. Of particular interest, we conclude that instead of beingsubstantially evolved as indicated by its classification, A7 IV-V,Altair is only barely off the zero-age main sequence and represents agood example of the difficulties rotation can introduce in theinterpretation of this part of the HR diagram.
| First sky validation of an optical polarimetric interferometer
Aims.We present the first lab and sky validation of spectro-polarimetricequipment put at the combined focus of an optical long-baselineinterferometer. We tested the polarimetric mode designed for the visibleGI2T Interferometer to offer spectropolarimetric diagnosis at themilliarcsecond scale. Methods.We first checked the wholeinstrumental polarization in the lab with a fringe simulator, and thenwe observed α Cep and α Lyr as stellar calibrators ofdifferent declinations to tabulate the polarization effects throughoutthe GI2T declination range. Results.The difference between bothlinear polarizations is within the error bars and the visibilitiesrecorded in natural light (i.e. without the polarimeter) for calibrationpurposes are the same order of magnitude as the polarized ones. Wefollowed the α Cep visibility for 2 h after the transit andα Lyr for 1.5 h and detected no decrease with hour angle due tothe fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetricCoudé trains of the GI2T is well-compensated by the fieldrotators, so the instrumental polarization is controlled over arelatively wide hour angle range (±2 h around the transit atleast). Such a polarimetric mode opens new opportunities especially forstudies of circumstellar environments and significantly enhances boththe potential of an optical array and its ability for accuratecalibration.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Detection of X-ray emission from β Pictoris with XMM-Newton: a cool corona, a boundary layer or what?
β Pictoris (HR 2020) is the most prominentprototype of stars with circumstellar disks and has generated particularinterest in the framework of young planetary systems. Given its spectraltype A5, stellar activity is not expected. Nevertheless, resonance linesof C iii and O vi typical for a chromosphere and transition region havebeen unambiguously detected with FUSE. We present results from anXMM-Newton observation of β Pic and find evidence for X-rayemission. In particular, we detected an emission of O vii at 21.6Å with the MOS detectors. These findings present a challenge forthe development of both stellar activity and disk models. We discuss andinvestigate various models to explain the observed emission includingthe presence of a cool corona and a boundary layer.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| A 2dF survey of the Small Magellanic Cloud
We present a catalogue of new spectral types for hot, luminous stars inthe Small Magellanic Cloud (SMC). The catalogue contains 4161 objects,giving an order-of-magnitude increase in the number of SMC stars withpublished spectroscopic classifications. The targets are primarily B-and A-type stars (2862 and 853 objects respectively), with oneWolf-Rayet, 139 O-type and 306 FG stars, sampling the main sequence to~mid-B. The selection and classification criteria are described, andobjects of particular interest are discussed, including UV-selectedtargets from the Ultraviolet Imaging Telescope (UIT) experiment, Be andB[e] stars, `anomalous A supergiants' and composite-spectrum systems. Weexamine the incidence of Balmer-line emission, and the relationshipbetween Hγ equivalent width and absolute magnitude for BA stars.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Characteristics and classification of A-type supergiants in the Small Magellanic Cloud
We address the relationship between spectral type and physicalproperties for A-type supergiants in the Small Magellanic Cloud (SMC).First, we construct a self-consistent classification scheme for Asupergiants, employing the calcium K to Hɛ line ratio as atemperature-sequence discriminant. Following the precepts of the `MKprocess', the same morphological criteria are applied to Galactic andSMC spectra, with the understanding that there may not be acorrespondence in physical properties between spectral counterparts indifferent environments. Then we discuss the temperature scale,concluding that A supergiants in the SMC are systematically cooler thantheir Galactic counterparts at the same spectral type, by up to ~10 percent. Considering the relative line strengths of Hγ and the CH Gband, we extend our study to F- and early G-type supergiants, for whichsimilar effects are found. We note the implications for analyses ofluminous extragalactic supergiants, for the flux-weightedgravity-luminosity relationship and for population synthesis studies inunresolved stellar systems.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| The field brown dwarf LP 944-20 and the Castor moving group
A reliable age estimation for the field brown dwarf LP944-20 is accomplished through the analysis of its kinematicproperties. The space velocities of this star strongly suggest itsmembership in the so-called Castor moving group. LP 944-20 can besensibly assumed to have the group's age, which is estimated to be ~320 Myr, and metal content, which is found to be roughly solar. Withthese new constrains and the available photometry and lithium abundance,current brown dwarf models are put to a test. Using the IR magnitudesand the lithium diagnostics, the models are able to provide a reasonabledescription of the brown dwarf's properties (to within a few sigma) butyield an age which is roughly 50% larger than our estimate. Possiblereasons for this discrepancy are discussed.
| A Far Ultraviolet Spectroscopic Explorer Survey of Late-Type Dwarf Stars
We describe the 910-1180 Å spectra of seven late-type dwarf starsobtained with the Far Ultraviolet Spectroscopic Explorer (FUSE)satellite. The stars include Altair (A7 IV), Procyon (F5 IV-V), αCen A (G2 V), AB Dor (K1 V), α Cen B (K2 V), ɛ Eri (K2 V),and AU Mic (M0 V). We present line identifications, fluxes, Dopplershifts, and widths. Doppler shifts are measured with respect toheliocentric wavelength scales determined from interstellar absorptionlines, and are compared with transition region line shifts seen inHubble Space Telescope (HST) ultraviolet spectra. For the warmer starsthe O VI lines extend the trend of increasing redshift with lineformation temperature, but for the cooler stars the O VI line redshiftsare essentially zero. The C III and O VI lines of most stars in thesample are best fit with two Gaussians, and we confirm the correlationof increasing importance of the broad component with increasing stellaractivity. The nonthermal velocities of the narrow component are subsonicand exhibit a trend toward larger velocities with decreasing surfacegravity, while the nonthermal velocities of the broad components show noobvious trend with stellar gravity. The C III and O VI lines of Altairshow unique broad horned profiles. Two flares were observed on AU Mic.One shows increasing continuum flux to shorter wavelengths, which weinterpret as free-free emission from hot plasma, and relatively narrow,redshifted C III and O VI emission. The other shows very broad lineprofiles.
| Limits on Chromospheres and Convection among the Main-Sequence A Stars
In deeply convective stars, the nonthermal energy required to heat thechromosphere ultimately is supplied by turbulent magnetoconvection.Because the early and middle A stars have very shallow convectivelayers, they are not expected to produce enough magnetoconvective powerto sustain luminous chromospheres or hot coronae. Here we describe asearch for chromospheric emission in the far-ultraviolet (905-1185Å) spectra of seven main-sequence A stars, based on observationsfrom the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope. Oursurvey spans the interval in effective temperature along the mainsequence over which powerful subsurface convection zones and hencechromospheric emission are expected to vanish. The presence or absenceof high-temperature emissions in our FUSE spectra therefore can be usedto identify the locus for the transition from convective to radiativeenvelopes-a change in stellar structure that is difficult to assess byother means. We present our observations and analysis of the subcoronalemission lines of C III λλ977, 1175 and O VIλλ1032, 1037, which bracket a range in formationtemperatures from 50,000 to 300,000 K. To supplement our FUSEobservations, we also report Goddard High Resolution Spectrographmeasurements of Si III λ1206 and H I Lyα λ1215,obtained from archival observations of the Hubble Space Telescope, aswell as X-ray measurements from previous ROSAT survey and pointedobservations. We detected C III and O VI emission features in the FUSEspectra of the coolest stars of our sample, at Teff<~8200K. When normalized to the bolometric luminosities, the detectedemission-line fluxes are comparable to solar values. We detected none ofthe hotter stars in our survey at Teff>=8300 K. Upperlimits on the normalized flux in some instances approach 40 times lessthan solar. Within an uncertainty in the effective temperature scale ofup to several hundred kelvins, our FUSE observations indicate that thetransition between convective and radiative stellar envelopes takesplace at, or very near, the point along the main sequence where stellarstructure models predict and, moreover, that the changeover occurs veryabruptly, over a temperature interval no greater than ~100 K in width.Our FUSE sample also includes two binary stars. In both cases, thenarrow UV line profiles we have observed suggest that thehigh-temperature emission is most likely associated with the late-typecompanions rather than the A stars themselves. Based on observationsmade with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer,operated for NASA by Johns Hopkins University under NASA contractNAS5-32985.
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| Observations of H3 in the Diffuse Interstellar Medium
Surprisingly large column densities of H+3 havebeen detected using infrared absorption spectroscopy in seven diffusecloud sight lines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR104, WR 118, and WR 121), demonstrating that H+3is ubiquitous in the diffuse interstellar medium. Using the standardmodel of diffuse cloud chemistry, our H+3 columndensities imply unreasonably long path lengths (~1 kpc) and lowdensities (~3 cm-3). Complimentary millimeter-wave, infrared,and visible observations of related species suggest that the chemicalmodel is incorrect and that the number density ofH+3 must be increased by 1-2 orders of magnitude.Possible solutions include a reduced electron fraction, an enhanced rateof H2 ionization, and/or a smaller value of theH+3 dissociative recombination rate constant thanimplied by laboratory experiments.
| Magnetic survey of bright northern main sequence stars
The first results of a systematic search for magnetic fields in thebrightest upper main sequence (MS) stars are presented. The main goal isto survey the stars with about the same detection limit and to improveexisting statistics of their magnetism. The target list contains 57upper MS stars and represents well B0.5-F9 stars. High-resolution Zeemanspectra were obtained for 30 stars of the list. The accuracy of themagnetic field measurements ranges from 20 to 300 G depending mainly onspectral class. In the majority of studied stars we did not detectmagnetic fields. In some stars we suspect the presence of a weakmagnetic field. These are the best candidates for more extensivestudies. A particular case is the star chi Dra where we probablydetected the global magnetic field. The longitudinal field strength isB_l= -54+/-12 G. Further observations of this star are needed to confirmthe detection and to ascertain if the magnetic field is variable withthe period of rotation. Based on observations collected at the 1 mtelescope of the Special Astrophysical Observatory (Nizhnij Arkhyz,Russia).
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| A 25 micron search for Vega-like disks around main-sequence stars with ISO
We present an ISO 25 mu m photometric survey of a sample of 81 nearbymain-sequence stars in order to determine the incidence of ``warm'' dustdisks. All stars were detected by ISO. We used an empirical relation toestimate the photospheric flux of the stars at 25 mu m. We find 5 stars(6%) with excess above the photospheric flux which we attribute to aVega-like disk. These stars show disk temperatures not warmer than 120K. Our study indicates that warm disks are relatively rare. Not a singlestar in our sample older than 400 Myr has a warm disk. We find an upperlimit of Mdisk = 2x 10-5 Moplus forthe mass of the disks which we did not detect. ISO is an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the United Kingdom) andwith the participation of ISAS and NASA.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Ages of A-Type Vega-like Stars from uvbyβ Photometry
We have estimated the ages of a sample of A-type Vega-like stars byusing Strömgren uvbyβ photometric data and theoreticalevolutionary tracks. We find that 13% of these A stars have beenreported as Vega-like stars in the literature and that the ages of thissubset run the gamut from very young (50 Myr) to old (1 Gyr), with noobvious age difference compared to those of field A stars. We clearlyshow that the fractional IR luminosity decreases with the ages ofVega-like stars.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
| Incidence and survival of remnant disks around main-sequence stars
We present photometric ISO 60 and 170 μm measurements, complementedby some IRAS data at 60 μm, of a sample of 84 nearby main-sequencestars of spectral class A, F, G and K in order to determine theincidence of dust disks around such main-sequence stars. Fifty starswere detected at 60 μm. 36 of these emit a flux expected from theirphotosphere while 14 emit significantly more. The excess emission weattribute to a circumstellar disk like the ones around Vega and betaPictoris. Thirty four stars were not detected at all; the expectedphotospheric flux, however, is so close to the detection limit that thestars cannot have an excess stronger than the photospheric flux densityat 60 μm. Of the stars younger than 400 Myr one in two has a disk;for the older stars this is true for only one in ten. We conclude thatmost stars arrive on the main sequence surrounded by a disk; this diskthen decays in about 400 Myr. Because (i) the dust particles disappearand must be replenished on a much shorter time scale and (ii) thecollision of planetesimals is a good source of new dust, we suggest thatthe rapid decay of the disks is caused by the destruction and escape ofplanetesimals. We suggest that the dissipation of the disk is related tothe heavy bombardment phase in our Solar System. Whether all starsarrive on the main sequence surrounded by a disk cannot be established:some very young stars do not have a disk. And not all stars destroytheir disk in a similar way: some stars as old as the Sun still havesignificant disks. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA. Tables 2, 3 and 4 are also available in electronicform at the CDS via anonymous ftp cdsarc.u-strasbg.fr (130.79.128.5)} orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/365/545
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Pozíciós és asztrometriai adatok
| Csillagkép: | Cefeusz |
| Rektaszcenzió: | 21h18m34.80s |
| Deklináció: | +62°35'08.0" |
| Vizuális fényesség: | 2.44 |
| Távolság: | 14.961 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 2.725 |
| V-T magnitude: | 2.458 |
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